It is known that conventional surface-treated steel strip usable for producing cans include electrolytically tin-plated steel strips, hereinafter referred to as tin plates, and electrolytically chromium-treated steel strips, hereinafter referred to as TFS-CTs.
In the above-mentioned types of steel strips, the tin plates are mostly typical surface-treated steel strip usable for producing cans and are most widely used for cans. That is, the tin plates, which may be painted or not painted, are widely used for producing food (sanitary) cans for containing fish, meat, soups or fruits therein, beverage cans for containing carbonated bevarages or fruit juices therein, 18 liter cans and pails, since the tin plates exhibit an excellent resistance to corrosion. The tin plates are used for producing not only there-piece cans but also two-piece cans which are widely used to contain beverages therein because the plated tin exhibits an excellent solid lubricating property.
TFS-CTs are mainly used to produce bevarage cans for containing bevarages therein having a relatively low corroding property, 18 liter cans and pails. Usually, TFS-CTs are used to produce three-piece cans but are not used to produce two-piece cans because the chromium-treated layer comprising mainly metallic chromium and hydrated chromium oxide is hard and brittle and, therefore, it is difficult to convert TFS-CTs into two-piece cans. Also even if TFS-CTs can be converted into two-piece cans, the resultant cans exhibit a remarkably decreased resistance to corrosion. When TFS-CTs are used to produce three-piece cans, the side seam bonding of the cans is usually effected by using a bonding material. If the side seam bonding of the cans is carried out by a seam welding process, it is necessary to remove the chromium-treated layer from TFS-CTs by means of grinding. Therefore, the seam welding process is usually not used in the case of TFS-CTs.
As stated above, the tin plates and TFS-CTs should be used for a specific use in which the characteristic properties thereof are beneficially utilized. For example, the tin plates are adequate for producing three-piece cans, which are produced by a conventional can-forming method which does not include the bonding method, and two-piece cans for containing therein materials, to be stably stored for a long period of time.
TFS-CTs are adequate for producing by the bonding method, three-piece cans for containing therein materials to be stored for a relatively short period of time.
Recently, since the price of tin has increased, it has become necessary to decrease the cost of production of tin plates and to conserve tin. For this purpose, attempts have been made to make the thickness of the tin layer in the tin plates smaller. A thin tin layer results in a decrease in the corrosion resistance of the tin plate. This diadvantage can be overcome by painting conventional plain tin plates. Accordingly, these is a tendency to replace conventional plain tin plates with painted tin plates. Also, TFS-CTs can be used in some cases in which conventional tin plates are used.
There is a tendency to increase the production of two-piece cans. Also, in the production of three-piece cans there is a tendency to use the seam welding method rather than the conventional soldering method.
In the case of recent tin plates, the weight of the plated tin is about 2.8 g/m.sup.2, which weight seems to be the lower limit of the weight of the plated tin layer necessary for enabling the layer to exhibit a satisfactory solid lubricating property in the formation of two-piece cans. However, weight of 2.8 g/m.sup.2 of the plated tin layer is not sufficient for enabling the layer to exhibit a satisfactory corrosion resistance. Accordingly, in the case of three-piece cans, the plated tin layer is usually painted before the can-forming procedure is carried out. Also, in the case of two-piece cans, the plated tin layer is usually painted after the can-forming procedure is carried out.
When TFS-CTs are used in place of tin plates, it is necessary that the entire surfaces of the TFS-CTs be painted. TFS-CTs are not adequate for producing cans for containing fruit juice therein since fruit juice is highly corrosive, or for containing materials therein which should be protected from iron ions derived from the can. Also, TFS-CTs are not adequate for producing two-piece cans which have been widely used in recent years. Therefore, the replacement of tin plates by TFS-CTs is not always acceptable.
As stated above, it is impossible to find a material for producing cans which is satisfactory in respect to corrosion resistance, paintability, formability, weldability and other properties necessary for cans.
For the purpose of decreasing the amount of tin to be plated on the steel strip surfaces, it has been attempted to place a plated nickel layer between the surface of the steel strip and the plated tin layer.
For example, Japanese Patent Application Publication No. 43-12246 discloses a tin-plated steel strip in which steel strip surfaces are base plated with a nickel or nickel-tin alloy layer in an amount of from 10 to 100 mg per m.sup.2 of each surface of the steel strip and then the surfaces of the base plated layers are plated with tin the plated steel strip is and reflowed. The tin-plated steel strip exhibits a satisfactory alloy-tin couple test value (ATC Value) which is a parameter of corrosion resistance. However, the above-mentioned reference contains no description concerning the paintability and weldability of the resultant tin-plated steel strip and the resultant tin-plated steel contains a considerably large amount of tin because both surfaces of the steel strip are plated with tin.
Japanese Patent Application Laid-open No. 49-119836 discloses a tin-plated steel strip in which a steel strip is base plated with a metallic nickel or copper layer having a thickness of from 0.005 to 1.0 micron and then surface plated with a tin layer having a thickness of from 0.1 to 2.0 microns without forming an alloy layer between the base layer and the surface layer. The above-mentioned reference states that the resultant tin-plated steel strip exhibits excellent paintability and corrosion resistance after being subjected to a draw and ironing (D.I) formation procedure. The reference, however, is completely silent as to the paintability and weldability when the tin-plated steel strip is used to produce three-piece cans. Also, the tin cost in the production of the tin-plated steel strip is high since the tin-plating is applied to both surfaces of the steel strip.
The above-mentioned two prior arts are effective for decreasing the amount of tin to be applied to the steel strip by preliminarily plating the steel strip with nickel or copper and the resultant products are usable for producing specific cans for limited use. The above-mentioned prior arts, however, failed to provide low cost materials for the production of cans which are satisfactory in respect to corrosion resistance, paintability formability, weldability and other properties necessary for producing cans.